Patent GB-786481-A (GB)
Title: Improvements in or relating to automatic electric control detection or measuring systems and instruments for use therein
Inventor: LOVEGROVE HENRY JOSEPH
IPC: G05F1/52
Filed: 1953-06-30
Abstract: 786,481. Voltage measurement; measuring bridges. SANGAMO WESTON, Ltd. June 2, 1954 [June 30, 1953], No. 18140/53. Addition to 700,643. Class 37. [Also in Groups XXXV and XXXVIII] An automatic control, detection, or measuring system comprises a sensitive D.C. moving coil instrument without restoring torque responsive to an input voltage or current to deflect a variable capacitor connected to detune a valve oscillator whereby the grid oscillatory signal amplitude is correspondingly varied and D.C. feedback is derived in response to such variations and applied to balance the variations of input voltage or current applied to the moving coil instrument; the system being applicable to the measurement of currents or voltages, the standardization of potentiometers, and the measurement of temperature in response to a thermocouple output. In Fig. 1 a voltage to be measured is connected over terminals 1, 2 in series with the moving coil system P of an instrument without restraining torque and in series with feed-back resistor R. Coil deflection varies a movable vane capacitor C1, detuning an oscillator comprising valve V, tuned anode circuit C2L2 and tuned grid circuit C1L1. When the input terminals are short-circuited the current in the coil P shunted across resistor R varies capacitance C1 so that the resultant change of P.D. across resistor n, due to the changing anode current, balances out the current in R while the application of a voltage to be measured initiates a further current in P, and the consequent variation of capacitor C1 adjusts the oscillator anode current until the voltage drop across R balances the applied voltage e to zeroize the current in coil P, so that the anode current measured by instrument K is proportional to the input. Fig. 2 shows a modification wherein the oscillator is Hartley connected, with the cathode of tetrode V1 returned to a tap on inductor L1  tuned by capacitor C1 and the anode retroactively coupled over capacitor C4 to the return of the tuned circuit. Capacitor C5 varied by the coil P controls the effective tapping point on L1 and thus the oscillatory amplitude, so that the system operates similarly to that of Fig. 1 to measure applied voltage at terminals 1, 2; an applied current being measurable (Fig. 3, not shown) by correcting the input terminals in parallel with the moving coil and inserting a series resistor between the coil and the junction of resistor R and instrument K in Fig. 2 which then indicates the input current at balance. Fig. 4 shows a circuit for standardizing a calibrating voltage derived from a source which energizes a potentiometer, comprising a variable resistor T and a fixed resistor R, across which the moving coil P is connected in series with a standard cell B4. Deflection of the coil varies capacitor C5 to regulate the oscillatory amplitude in, and thus the D.C. consumed by, a triode V2 in Hartley connection, the voltage drop across the anode load R1 biasing a triode V3 in series with the voltage supply at terminals 5, 6 for adjustment so that the voltage developed across R is maintained equal to the standard cell voltage irrespective of supply variations and the calibrating voltage at terminals 3, 4 is maintained constant at a valve set by resistor T. Fig. 5 shows a circuit measuring the voltage output of a temperature measuring thermojunction W having the input terminals 1, 2 connected in series with coil P across the output diagonal of a compensating bridge R2, R3, R4, R5 constituting the " cold " thermojunction, whose input is connected across stabilized voltage supply B5 in series with a resistor q, and whose component resistor R2 is connected in place of resistor R in the circuit of Fig. 2. The resistors R2, R3, R4 are of a temperature invariable material while resistor R5 is of a temperature variable metal such as nickel or copper, and the thermocouple leads may also be of temperature invariable material. Initially, with the input terminals shortcircuited, there is no current in P, and capacitor C5 assumes a value controlling the oscillator amplitude and anode current so that zero current flows in instrument K. Energization of coil P due to the connection of the thermojunction varies capacitor C5 and the anode current of the oscillator until the bridge output voltage balances the thermo-junction voltage and the value of anode current measured by instrument K is proportional to the junction temperature; the bridge supplying compensating voltage which is a function of the cold junction temperature.